(1) Field of the Invention
The present invention relates to transducer assemblies and to apparatus employing same for achieving efficient combustion of fuels. An example of same is found in the U.S. Pat. to H. L. Berger, No. 3,861,852, issued Jan. 21, 1975.
(2) Description of the Prior Art
When designing ultrasonic transducer assemblies such as those employed in apparatus for achieving combustion of fuels, a theoretical model for the ultrasonic horn is used in the developmental stage. The theoretical model is that of a one dimensional transmission line.
In the actual operating environment, however, deviations from the theoretical model are introduced. The deviations are due to, among other things: the finite dimensions of the sections of the horn setting up modes other than longitudinal, e.g. expansion in a transverse direction; clamping means; sealing means; physical mismatch between component parts (planarity); etc.
The introduction of the deviation into the theoretical model normally produces internal losses in the transducer assembly and thus reduces Q, the mechanical merit factor.
The approach used in designing such prior art transducer assemblies so as to achieve maximum Q has been to: treat the entire assembly as a theoretical structure; choose the vibration frequency at which the structure is in resonance; provide an ultrasonic horn, according to a theoretical model whose size is such as to provide the resonance condition; and, utilize materials and associated hardware such as fuel supply means, clamp means, seals, etc., of such type and so positioned as to minimize losses inherent in the deviation from the theoretical model.
The prior art design approaches have failed to achieve maximum Q for a number of reasons: inappropriate design (deviations from the theoretical model); and, poor acoustical coupling between the center electrode and the piezoelectric crystals of the driving element and between the driving element crystals and adjacent ultrasonic horn sections caused either by imperfect machining of the crystals or by the presence of contaminants between the mating surfaces.
A second problem associated with transducer assemblies of the type used in apparatus for achieving combustion of fuels is the non-uniform delivery of fuel to the atomizing surface with consequent non-uniform distribution of fuel from same. It has been discovered that with such prior art assemblies, fuels which have low surface tension as, for example, hydrocarbon fuels, begin to atomize within the fuel passage leading to the atomizing surface. This premature atomization creates bubbles within the fuel passage. The bubbles eventually work their way to the atomizing surface, but their arrival at the atomizing surface results in a temporary interruption in fuel flow to portions of the surface and, as a result, non-uniform distribution of fuel over the surface. The bubble remains intact for a short period of time on the atomizing surface and thus the surface area beneath the bubble during the interval is not wet with fuel.
A third problem associated with transducer assemblies of the type used in apparatus for achieving combustion of fuels is that the fuel, once delivered to the atomizing surface, even if delivered uniformly, is not distributed or atomized from same uniformly. It has been discovered that one of the reasons for non-uniform distribution is the flexing action of the atomizing surface itself, characteristic of the prior art structure.
A fourth problem associated with prior art transducer assemblies is lack of efficiency. Briefly stated, in an ultrasonic fuel atomizer a film of fuel is injected at low pressure onto an atomizing surface and vibrated at frequencies in excess of 20 kHz in a direction perpendicular to the atomizing surface. The rapid motion of the plane surface sets up capillary waves in the liquid film. When the amplitude of wave peaks exceeds that required for stability of the system, the liquid at the peak crests breaks away in the form of droplets.
The smaller the droplet size the greater the fuel-air interface for a given volume of fuel. The increased fuel-air interface allows better utilization of primary combustion air resulting in low-excess air combustion, a desirable feature from an efficiency standpoint.
Going one step further, for a given fixed volume flow rate of fuel reaching the atomizing surface, the thinner the film, the more surface area will be involved in the atomizing process. This allows for greater atomizing capacity. It has been discovered that prior art transducer assemblies have been limited in this respect, however, due to the fact that the fuel fed to the atomizing surface does not cover the entire surface before atomization occurs. Additionally the surface tension associated with smooth metallic atomizing surfaces give rise to a tendency for not wetting the entire surface.